42. What does the"law of nature"mean? You can find the answer in the passage in (   ) . 

• A、the second sentence
  
  

• B、the fifth sentence
  
  

• C、the seventh sentence
  
  

• D、the forth sentence
  
  

10. Only then (   ) that he had left his key on his office. 

• A、he realized
  
  

• B、did he realize
  
  

• C、had he realized
  
  

• D、he had realized
  
  

53 .(    )